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Here's a crash course: "I tell my friends to think of metabolism as a car engine that's always running. It needs gas just to turn over, and the faster it's asked to go, the more gas it demands. How much fuel your engine requires depends on how you're built: Leaner, more muscular people burn more calories, even when they're not active, and thus need to eat more. The number of calories you'd burn if your engine sat in idle all day—in other words, if you stayed in bed—is called your 'resting' or basal metabolic rate (BMR), and it's key to figuring out how to maintain your weight or lose pounds. [Use the worksheet on the next page to determine how many calories you should be eating.] But that doesn't mean you're stuck with the metabolism you have now. Your BMR changes throughout your life, depending on your age, weight, and activity level, which means you can move your engine closer to being a powerful V8 if you become more fit." —Michael D. Jensen, M.D., director of the Department of Medicine Obesity Research Treatment Program at the Mayo Clinic in Rochester, MN

Do what you love: "A lot of people turn to high-intensity interval training (HIIT) thinking it will boost their metabolism. The idea: Short, intense bursts of exercise torch a huge amount of calories, and because HIIT includes resistance moves, it also builds muscle that will provide a consistent burn even after the class is over. But it's much more important to find a workout you'll love, because staying active is crucial for your metabolism. A good friend decided to try ballroom dancing and got so into it that she and her husband now compete. There are all sorts of activities like this that are fun and social and burn a surprising number of calories: racquetball, salsa, an adult soccer or basketball league. Think about what you enjoyed as a kid—chances are you'll still like it!" —Andrea Dunaif, M.D., professor of medicine at Northwestern University Feinberg School of Medicine in Chicago

Nutrients are best in threes: "I have a friend who gained a lot of weight and just couldn't get it off. Turns out, she was relying on a lot of no-fat and low-fat foods. That sounds healthy, but these products often contain added sugar, which causes your metabolism to spike and then plummet, making you crave even more sugar. The secret to keeping your metabolism stable is to eat a balanced diet of lean protein, good carbs (so: fiber-rich whole grains, fruit, and veggies), and healthy fats at every meal, and especially at breakfast. I told my friend to start her day with high-fiber cereal, plain yogurt, and a handful of walnuts, or a hard-boiled egg and a slice of whole-grain toast topped with avocado. Then eat this same balance of protein, carbs, and fat for lunch and dinner. She felt full between meals, had fewer cravings, and because good fats and fiber work in tandem to boost metabolism, she was able to drop the extra pounds and keep them off. It's a strategy I've seen work over and over again in my practice." —Eugenia Gianos, M.D., codirector of the Center for the Prevention of Cardiovascular Disease at New York University Langone Medical Center

Make sleep a priority: "Sleep is incredibly important for metabolism. Research suggests that people who aren't well-rested have lower levels of ghrelin (the satiety hormone) and higher levels of leptin (the hunger hormone). In other words, they're hungrier and don't feel full as fast. If you can't sleep a full eight hours, remember that quality is as important as quantity. I tell my friends to practice good sleep hygiene: Keep your bedroom cool and dark and free of distractions like TVs, smartphones, and tablets. Use your bed only for sleep and sex. Finally, if you're still struggling to doze off and stay that way, get back up. Lying in bed awake only creates anxiety and stress, which in turn makes it more difficult to sleep." —Ari Laliotis, M.D., cofounder of the San Diego Center for Sleep Medicine

Don't be a statistic: "The thyroid gland is the master control of metabolism, and since one in eight women will develop a thyroid disorder, it's imperative to do what you can to take care of it. Most risk factors (genetics, stress) are out of your control, but you can pay attention to your diet. The most important element for producing thyroid hormone is iodine, and while most people get enough of it, we've started to see more cases of iodine deficiency as Americans have steered away from processed foods and salt. When you do use salt, make sure it's iodized—most sea salt is not—and if you're pregnant, add a multivitamin that provides 180 to 250 mcg of iodine a day. For some people, it can make a big difference: I know a woman who was headed in the direction of developing hypothyroidism, but she really, really didn't want to take medication. I suggested she cook more at home and also try things like meditation to ease her stress, and her thyroid levels improved. For others, though, medication is necessary, so if you're experiencing symptoms of an underactive thyroid, like weight gain, fatigue, heavy periods, or fewer bowel movements, see your doctor." —Laura Ryan, M.D., assistant professor of endocrinology, diabetes, and metabolism at the Ohio State University Wexner Medical Center in Columbus

Bust a move (any move): "A friend was always beating herself up for not getting to the gym. But she also had a desk job, and research suggests that even a daily workout can't negate the detrimental effects of sitting for more than eight hours per day. I told her that her metabolism would be more efficient if she consistently kept moving in little ways. Walking around the office is great, but everyday movements like folding laundry, fidgeting in your seat, and chewing gum can help too. Experts refer to those things as NEAT: non-exercise activity thermogenesis. Thermogenesis is the process that produces heat in our bodies, making us expend more energy and increasing our BMR. My friend tried it and immediately noticed that she not only had more energy, but also slept better at night." —Jessica Bartfield, M.D., internist and weight-loss specialist at Loyola University Health System in Chicago

Have your wine: "A friend in her 30s recently told me, 'I don't know what's going on. When I gained a few pounds, I used to be able to just not drink wine for a week and get it off. Now the weight won't budge.' Two things are at work here: As you age, your body starts to secrete hormones that prevent you from feeling full, so you unknowingly eat more. At the same time, your muscles become less efficient at burning calories and especially at breaking down glucose (or sugar). I told her to stop stressing about the vino and instead cut back on white foods: bread, pasta, rice. Unlike whole grains, refined carbs trigger a large release of insulin, which may then store extra fat. As for the wine, red contains resveratrol, which research shows may help you better metabolize the sugar you do eat." —Laura Ryan, M.D.